EP1437613A1 - Optical fiber with a Bragg grating directly written through a cladding coating - Google Patents

Abstract

The coating (4) material contains an inorganic substance (6) immiscible with an organic substance (5) in it and distributed homogeneously in the material. An Independent claim is included for the manufacture of an optical fibre.

Description

The present invention relates to an optical fiber with at least one Bragg grating obtained by direct inscription through the coating covering the sheath as well as a process for manufacturing an optical fiber with at least one Bragg grating.

In known manner, Bragg grating optical fibers have a germanium doped silica core successively covered a silica sheath and a coating of a material chosen to be transparent and heat resistant to radiation from the network generally emitted by a laser and of the ultraviolet (UV) type. In this way, the Bragg network is registered in the core and / or the sheath directly through this coating.

The document entitled "Grating writing though fiber coating at 244 nm and 248 nm" Chao et al, Electronics Letters, 27 ^th May, 1999 Vol 35, n ° 11, pp924-925 thus discloses a fiber to a Bragg grating obtained by a direct registration of the network in the core of the fiber through its coating.

The coating is made of silicone which has a transmittance equal to Around 90%, in particular with two conventional UV registration rays: 244 nm and 248 nm. The thermal resistance of this silicone is also validated by passing through an oven at 300 ° C for 3 minutes.

The smallest Bragg grating described is 1 cm long and has a low contrast - that is to say a variation in refractive index - of 2 10 ⁴ , corresponding to a reflectivity of 92%.

In addition, registration is carried out using a scanning of the laser beam which requires complex equipment.

The mechanical properties of this silicone are not satisfactory, especially in terms of longevity, the fiber being able for example to deteriorate during storage. In addition, this silicone is not sufficiently resistant to water which is critical in submarine connections.

This silicone is the product RTV615 from the company General Electric obtained from a bicomponent composition curable at temperature ambient, six to seven days at 25 ° C, and comprising two known manner silicone precursors, to be mixed just before application to the sheath the fiber. The life time of this mixture is four hours.

This composition, slightly viscous, proves difficult to apply. form. As soon as the two precursors are brought into contact, the viscosity of the mixture changes very quickly which means that the thickness of the coating, on average equal to 60 µm, is not constant over the entire length of the fiber. Moreover, this fiber can not be wound on itself quickly because the time of hardening is on the order of several days. The contact of two parts of the fiber during drying causes the coating of these to stick between they.

The object of the invention is to overcome the aforementioned problems in providing optical fiber to at least one Bragg grating obtained by direct writing through the coating covering the optimized sheath in terms of thermal and photochemical resistance and transparency UV type inscription radiation. This fiber must have good mechanical properties, improved longevity, and the network (s) Bragg present optical properties adjusted according to applications sought.

To this end, the invention provides an optical fiber with at least one Bragg grating comprising a heart successively surrounded by a sheath and a coating, said fiber being obtained by direct inscription of said network in the core and / or the sheath through the coating of a material containing an organic substance substantially transparent to a radiation from said ultraviolet type network, characterized in that the material of said coating contains an inorganic substance not miscible with the organic substance and distributed approximately homogeneous in said material.

The material according to the invention is a homogeneous material in the sense that it has substantially the same properties at all points of the material and isotropic in the sense that it has the same properties in all dimensions of space.

The presence of both the inorganic substance and the substance organic does not cause chemical or physical reactions undesirable leading to a decrease in physical performance nor even evolution of the structure over time. On the contrary, the material according to the invention has the cumulative properties of each of the two substances.

The organic substance is chosen to give the material of the coating its transparency to ultraviolet radiation. The substance organic may contain one or more of the following chemical bonds: C-C, C-Si, C-I, C-H, C-O, O-H, Si-O, Si-H, C-F, C-CI, Ge-C, Ge-Si these have no significant absorbance in the ultraviolet of wavelength greater than or equal to 240 nm.

And this organic substance can be free of nuclei aromatic, of unsaturated conjugations, these elements absorbing strongly ultraviolet radiation.

For example, additives generally contain such groupings and are to be avoided because of their opacity.

Similarly, metal-based and especially platinum-based catalysts such as Pt (AcAc) ₂ , PtCpMe ₃ used to obtain a polymer by hydrosylilation are to be avoided as far as the presence of metals reduces the longevity of the fiber.

The inorganic substance is chosen to impart material to the coating according to the invention improved mechanical properties (storability, resistance to fatigue, traction, impact) an improvement grip on the sheath, water and solvent tightness reinforced organic and better viscosity control to facilitate coating formation and better control its thickness over the entire fiber length.

In addition, this inorganic substance does not alter the transparency but increases the thermal and photochemical resistances of the coating. This thus avoids any degradation of the fiber induced by the inscription radiation even at high fluences, for example greater than 1000 J.cm- ² .

In addition, the surface condition of the coating material in particular its roughness is better controllable and will improve the transmittance profile of the coating, which can in particular reduce the duration Registration.

A wide range of properties can be obtained by changing the nature, composition, proportion of the inorganic substance.

Advantageously, the inorganic substance can be chosen among a mineral filler, a reinforcing agent and preferably contains particles with an average particle size of less than one twentieth of the wavelength of said writing radiation, for example between 190 nm and 400 nm, and in particular at standard values 244 nm, 248 nm, 266 nm, 302 nm, 355 nm or at the wavelengths of UV lasers available on the market.

This limitation of the average particle size makes it possible to avoid any diffraction and other distortion phenomenon of the inscription radiation and therefore to obtain an ad hoc Bragg network.

According to one characteristic, the inorganic substance contains silica which preferably is pyrogenic.

The choice of silica is particularly suitable when the substance organic mainly contains silicone.

The coating material can include any proportions to improve the physical properties of the coating.

According to a preferred characteristic, the coating material can include up to 30% by mass of the inorganic substance and when the organic substance contains silicone, the material includes between 1.2 and 5% by mass of the inorganic substance and preferably about 2.75 % by mass.

The Bragg grating can have a contrast of 7.4 10 ^-4 for a grating length of 1 mm and a contrast of 2.3 10 ^-4 for a grating length of 5 mm. These networks can be used for gain equalizing networks, reflective networks for routing functions, for laser resonators or chromatic dispersion compensators.

The invention naturally applies to an optical device incorporating an element of a material as defined above.

The invention is naturally suitable for the manufacture of devices containing a fiber as defined above. We can quote by example of optical filters, demultiplexers, compensators of dispersion, including gain equalizing filters and all especially the slope equalizing filters (PTEQ for Passive Tilt EQualize in English).

This material can also be used for any element other than fiber as long as UV transparency and / or thermal resistance and / or chemical is useful to him. For example, this element can be an adhesive, a phase mask or optical component.

• la préparation d'un mélange durcissable contenant ladite substance inorganique et une substance organique liquide contenant au moins un précurseur de polymère durcissable,
• l'application du mélange durcissable sur ladite gaine en monocouche,
• le durcissement du mélange durcissable de façon à former ledit matériau.

The invention also provides a method of manufacturing the fiber with at least one Bragg grating as defined above, a method comprising a step of forming said coating on the sheath and a step of registering said network through said coating in the core. and / or the sheath using a source of the inscription radiation. The coating formation step includes:

• the preparation of a curable mixture containing said inorganic substance and a liquid organic substance containing at least one precursor of curable polymer,
• applying the curable mixture to said monolayer sheath,
• curing the curable mixture so as to form said material.

The addition of the inorganic substance makes the mixture more viscous therefore applicable.

By choosing a thermosetting polymer precursor, it suffices simply integrate one or more ovens into the fiber-drawing tower to obtain the material .

By choosing a photocurable polymer precursor, it suffices simply integrate one or more UV sources into the fiber-drawing tower to get the material.

The registration step of said network can preferably be carried out in static and not by sweeps as in the prior art.

Preferably, the polymer precursor is chosen from a thermosetting silicone precursor and a silicone precursor curable.

• la figure 1 montre le profil de transmittance T (exprimée en %) en fonction de la longueur d'onde (exprimée en nm) d'un substrat en silice revêtu d'un matériau d'épaisseur 60µm selon l'invention,
• la figure 2 représente la fibre optique obtenue après formation d'un revêtement d'une gaine d'une fibre optique à photoinscrire conforme à l'invention,
• la figure 3, représente une fibre à réseau de Bragg dans le mode de réalisation préféré de l'invention.

The features and advantages of the invention will become clear on reading the description which follows, given by way of illustrative and nonlimiting example and made with reference to the appended figures in which:

• FIG. 1 shows the transmittance profile T (expressed in%) as a function of the wavelength (expressed in nm) of a silica substrate coated with a material with a thickness of 60 μm according to the invention,
• FIG. 2 represents the optical fiber obtained after forming a coating of a sheath of an optical fiber to be photo-registered according to the invention,
• Figure 3 shows a Bragg grating fiber in the preferred embodiment of the invention.

The invention lies in the choice of a material containing a organic substance and an inorganic substance immiscible with organic substance and distributed in a substantially homogeneous way in the material, substances selected to form a coating of a fiber optical to one or more Bragg gratings, that is to say to obtain quickly coating, allowing direct registration of network (s) of Bragg, giving the fiber good mechanical properties.

• une substance inorganique, jusqu'à 30 % en masse, qui contient de préférence des particules de granulométrie moyenne inférieure à un vingtième de la longueur d'onde du rayonnement d'inscription, choisie par exemple égale à 248 nm,
• une substance organique liquide contenant au moins un précurseur de polymère durcissable choisi de préférence parmi les précurseurs de polymère thermodurcissables, les précurseurs de polymère photodurcissables

The step of forming this coating firstly comprises the preparation of a curable mixture containing:

• an inorganic substance, up to 30% by mass, which preferably contains particles with an average particle size less than one twentieth of the wavelength of the inscription radiation, chosen for example equal to 248 nm,
• a liquid organic substance containing at least one curable polymer precursor preferably chosen from thermosetting polymer precursors, photocurable polymer precursors

Preferably, the inorganic substance is fumed silica and the polymer precursor is a heat-crosslinkable silicone precursor for example the two-component product LSR1551 from the company Dow Corning.

In this configuration, the mixture contains between 1.2 and 5% in mass of the fumed silica and preferably about 2.75% by mass.

This curable mixture of controlled viscosity and equal to 6000 mPa.s at 25 ° C is then applied in a monolayer of thickness equal to 60µm using a coating tower on a fiber sheath.

Passage through ovens at 100 ° C. followed by annealing at 150 ° C. leads to the formation of said material. On exit, the coating looks transparent and non-sticky.

The material obtained is free of aromatic rings, of conjugated unsaturations and is transparent and resistant to the radiation of ultraviolet type of Bragg network registration.

Figure 1 shows the transmittance profile T (expressed in%) in function of the wavelength (expressed in nm) of a silica substrate coated with 60 μm of the material which has just been described.

We see in this figure 1 that the transmittance T exceeds 90% between 215 to 500 nm or over a wide wavelength range.

60 µm is the current standard thickness of fiber coating at minus a Bragg grating.

• pour 20 µm : 99%,
• pour 30 µm : 98%,
• pour 124 µm : 91,5%,
• pour 240 µm : 87%.

For other layer thicknesses, the transmittances are, at 248 nm, as follows:

• for 20 µm: 99%,
• for 30 µm: 98%,
• for 124 µm: 91.5%,
• for 240 µm: 87%.

• Ebecryl 350, Ebecryl 1360 de la société UCB,
• UVS-500 de la société Croda,
• Additive 97-168, additive 97-169, additive 99-622 Rahn- de la société Genomer,
• Rad 2100, Rad 2200, Rad 2500, rad 2600, Rad 2700 (Tego chemie) DMS-U22 de la société Gelest,
• les produits de la gamme silcolease UV de la société Rhodia,
• DMS-R01, DMS-R05, DMS-R18, DMS-R22, DMS-R31 (Gelest)
• le Poly[dimethylsiloxane-co-[2-(3,4-epoxycyclohexyl) ethyl] methylsiloxane] de la société Aldrich.

In a variant, it is possible to choose a photocrosslinkable silicone precursor, by radiation of wavelength distinct or not from that of inscription insofar as the silicone becomes transparent after its crosslinking, and for example chosen from the list of products following:

• Ebecryl 350, Ebecryl 1360 from UCB,
• UVS-500 from Croda,
• Additive 97-168, additive 97-169, additive 99-622 Rahn- from Genomer,
• Rad 2100, Rad 2200, Rad 2500, rad 2600, Rad 2700 (Tego chemie) DMS-U22 from the company Gelest,
• the products of the UV silcolease range from Rhodia,
• DMS-R01, DMS-R05, DMS-R18, DMS-R22, DMS-R31 (Gelest)
• Poly [dimethylsiloxane-co- [2- (3,4-epoxycyclohexyl) ethyl] methylsiloxane] from the company Aldrich.

It is important to choose a photoinitiator (if necessary) no absorbent in writing lengths after passing through an oven ad hoc ultraviolet.

The organic substance can also be a polymer other than a silicone as long as it is transparent to UV.

• un polyacrylate tels que les produits Repolem 1137, Repolem PHE05 de la société ATOFINA,
• un polymère thermoplastique choisi parmi les polyfluorures de vinylidène (PVDF) et les copolymères PVDF et d'hexafluoropropène (HFP) tels que les produits Kynar de la société ATOFINA,
• les polyéthylènes glycol acrylate, les polyuréthanes aliphatiques acrylates.

Among the polymers free of aromatic rings, of conjugated unsaturations, there may be mentioned:

• a polyacrylate such as the products Repolem 1137, Repolem PHE05 from the company ATOFINA,
• a thermoplastic polymer chosen from polyvinylidene fluorides (PVDF) and copolymers PVDF and hexafluoropropene (HFP) such as the Kynar products from the company ATOFINA,
• polyethylene glycol acrylates, aliphatic polyurethanes acrylates.

The fiber according to the invention obtained after the formation of the coating is presented in longitudinal view in Figure 2 where we see this optical fiber 1 which has a core 2 of germanium doped silica, covered successively a sheath 3 of silica and a coating of the material 4.

This fiber can be coiled on a reel for hydrogenation without not break and can be stored for several months on this spool.

The silicone 5 (see the enlarged area in Figure 2) forms the matrix of the coating, and the fumed silica 6, corresponds to a reinforcing element.

The fiber 1 Bragg network registration inscription is performed statically. The influence of registration, for example from a UV laser source is of wavelength, for example chosen at 248 nm.

• densité d'énergie moyenne : 70 mJ/cm,
• durée d'inscription : 12 minutes,
• fréquence : 20 Hz,
• soit une fluence totale de : 1010 J/cm².

The registration conditions are as follows:

• average energy density: 70 mJ / cm,
• registration duration: 12 minutes,
• frequency: 20 Hz,
• i.e. a total fluence of: 1010 J / cm ² .

• longueur d'inscription : 1 mm,
• contraste : 7,4 10^-4,
• profondeur : 6 dB à 1571.nm.

The characteristics of the Bragg network are as follows:

• inscription length: 1 mm,
• contrast: 7.4 10 ^-4 ,
• depth: 6 dB at 1571.nm.

The contrast achieved is very high without degradation of the coating even at a very high chosen fluence: the material present thus very high photochemical and thermal resistances.

• densité d'énergie moyenne : 45,5 mJ/cm,
• durée d'inscription : 29 minutes,
• soit une fluence totale de 1580 J/cm²,
• fréquence : 20Hz.

In a variant, for example for the use of Bragg grating fiber in gain equalizing arrays, reflective arrays for routing functions, for laser resonators or chromatic dispersion compensators the registration conditions are as follows: :

• average energy density: 45.5 mJ / cm,
• registration duration: 29 minutes,
• or a total fluence of 1580 J / cm ² ,
• frequency: 20Hz.

• longueur d'inscription : 5 mm,
• contraste : 2,3 10^-4,
• profondeur : 11,8 dB à 1559 nm.

The characteristics of the Bragg network are then as follows:

• inscription length: 5 mm,
• contrast: 2.3 10 ^-4 ,
• depth: 11.8 dB at 1559 nm.

Figure 3 is a longitudinal view of a fiber 1 'Bragg grating 11 in a preferred embodiment of the invention.

This fiber 1 'thus obtained is for example intended to be incorporated in an optical device (not shown), for example of the type signal filter or repeater in long optical transmissions distances.

In a variant, in order to increase the refractive index of the coating above 1.45 (which corresponds to the index of doped silica germanium), additives with an adequate refractive index are added to the curable mixture for making an oblique Bragg grating fiber (Slanted Bragg Grating in English) effective.

Of course, the invention is not limited to the embodiment which has just been described.

The fiber can comprise several Bragg gratings, the length of the or Bragg gratings is to be adapted according to the applications sought. The photo-registration conditions can be modified by example to increase contrast and / or depth.

Finally, we can replace any means with equivalent means without depart from the scope of the invention.

Claims (9)

Optical fiber (1 ') with at least one Bragg grating (11) comprising a core (2) successively surrounded by a sheath (3) and a coating (4), said fiber being obtained by direct registration of said network in the core and / or the sheath through the coating of a material containing an organic substance (5) substantially transparent to an inscription radiation of said ultraviolet type network, characterized in that the material of said coating contains an inorganic substance (6) immiscible with the organic substance and distributed in a substantially homogeneous manner in said material. Optical fiber with at least one Bragg grating according to claim 1, in which the inorganic substance is chosen from a filler mineral, a reinforcing agent and preferably contains particles with an average particle size of less than one twentieth of the length wave of said inscription radiation. Optical fiber to at least one Bragg grating according to one of the claim 1 or 2, wherein the inorganic substance is preferably fumed silica. Optical fiber to at least one Bragg grating according to one of the claims 1 to 3, wherein said material comprises up to 30% by mass of the inorganic substance and, when the substance organic contains silicone, the material includes between 1.2% and 5% by mass of the inorganic substance and preferably about 2.75% en masse. Optical fiber with at least one Bragg grating according to one of Claims 1 to 4, in which the Bragg grating has a contrast of 7.4 10 ^-4 for a grating length of 1 mm and a contrast of 2.3 10 ^-4 for a network length of 5 mm. Optical device incorporating a Bragg grating fiber such as defined according to one of claims 1 to 5. Method of manufacturing an optical fiber with at least one Bragg grating defined according to one of claims 1 to 5, method comprising a step of forming said coating on the sheath and a step of writing said grating through said coating in the core and / or the sheath using a source of the inscription radiation, in which the step of forming the coating comprises: the preparation of a curable mixture containing said inorganic substance and a liquid organic substance containing at least one precursor of curable polymer, applying the curable mixture to said monolayer sheath, curing the curable mixture so as to form said material. Method of manufacturing an optical fiber with at least one network of The Bragg of claim 7, wherein the step of enrolling said Bragg grating is performed statically. Method of manufacturing an optical fiber with at least one network of Bragg according to one of claims 7 or 8, in which the precursor of polymer is chosen from a silicone precursor thermosetting and a photocurable silicone precursor.

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